Sensory Physiology
215
acuity. Thus, a great deal of information processing takes place
at this early stage of the sensory pathway.
The axons of the ganglion cells form the output from the
retina—the optic nerve, which is cranial nerve II (
Figure 7–29
).
The two optic nerves meet at the base of the brain to form the
optic chiasm,
where some of the fi bers cross and travel within
optic tracts
to the opposite side of the brain, providing both
cerebral hemispheres with input from each eye.
Parallel processing of information continues all the way
to and within the cerebral cortex to the highest stages of visual
neural networks. Cells in this pathway respond to electrical sig-
nals that are generated initially by the photoreceptors’ response
to light. Optic nerve fi bers project to several structures in the
brain, the largest number passing to the thalamus (specifi cally to
the lateral geniculate nucleus of the thalamus, see Figure 7–29),
where the information from the different ganglion cell types
is kept distinct. In addition to the input from the retina, many
neurons of the lateral geniculate nucleus also receive input
from the brainstem reticular formation and input relayed back
from the visual cortex. These nonretinal inputs can control
the transmission of information from the retina to the visual
cortex and may be involved in our ability to shift attention
between vision and the other sensory modalities.
The lateral geniculate nucleus sends action potentials to
the visual cortex, the primary visual area of the cerebral cortex
(see Figures 7–14 and 7–29). Different aspects of visual infor-
mation are carried in parallel pathways and are processed simul-
taneously in a number of independent ways in different parts
of the cerebral cortex before they are reintegrated to produce
the conscious sensation of sight and the perceptions associated
with it. The cells of the visual pathways are organized to handle
information about line, contrast, movement, and color. They
do not, however, form a picture in the brain. Rather, they form
a spatial and temporal pattern of electrical activity.
We mentioned that a substantial number of fi bers of the
visual pathway project to regions of the brain other than the
visual cortex. For example, visual information is transmitted
to the
suprachiasmatic nucleus,
which lies just above the
optic chiasm and functions as part of the “biological clock.”
Information about cycles of light intensity is used to entrain
this neuronal clock to a 24-hour day. Other visual informa-
tion passes to the brainstem and cerebellum, where it is used
in the coordination of eye and head movements, fi xation of
gaze, and change in pupil size.
Color Vision
The colors we perceive are related to the wavelengths of light
that the pigments in the objects of our visual world refl ect,
absorb, or transmit. For example, an object appears red because
it absorbs shorter wavelengths, which would be perceived as
blue, while it refl ects the longer wavelengths, perceived as red,
to excite the photopigment of the retina most sensitive to red.
Light perceived as white is a mixture of all wavelengths, and
black is the absence of all light.
Color vision begins with activation of the photopigments
in the cone photoreceptor cells. Human retinas have three
kinds of cones—one responding optimally at long wavelengths
(“red” cones), one at medium wavelengths (“green” cones),
and the other stimulated best at short wavelengths (“blue”
cones). Although each type of cone is excited most effectively
by light of one particular wavelength, there is actually a range
of wavelengths within which a response will occur. Thus, for
any given wavelength, the three cone types are excited to dif-
ferent degrees (
Figure 7–30
). For example, in response to
light of 531-nm wavelengths, the green cones respond maxi-
mally, the red cones less, and the blue not at all. Our sensa-
tion of the shade of green at this wavelength depends upon
the relative outputs of these three types of cone cells and the
comparison made by higher-order cells in the visual system.
The pathways for color vision follow those that Figure
7–29 describes. Ganglion cells of one type respond to a broad
Left eye
Right eye
Optic
chiasm
Optic
nerve
Optic
tract
Occipital
lob
e
Lateral
geniculate
nucleu
s
Visual corte
x
Figure 7–29
Visual pathways viewed in cross section from above.
Figure 7–29
physiological
inquiry
Three patients have suffered destruction of different portions
of their visual pathway.
Patient 1 has lost the right optic tract,
patient 2 has lost the nerve fi bers that cross at the optic chiasm,
and patient 3 has lost the left occipital lobe.
Draw a picture of
what each person would perceive through each eye when looking
at a white wall.
Answer can be found at end of chapter.
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